NIR light-steered magnetic liquid marbles with switchable positive/negative phototaxis

This work has demonstrated a magnetic liquid marble of switchable positive/negative phototaxis and excellent mechanical stability by encapsulating water droplet with oleic acid modified Fe3O4 nanoparticles (Fe3O4@OA NPs). The as-constructed marble can reach a height up to 4.7 mm, indicating an effective surface tension of ∼54 mN m−1. The marble on water surface can be steered by NIR light as well as magnetic field due to the photothermal effect and superparamagnetic feature of the Fe3O4@OA NPs. Its light-steered motion stems from the photothermal effect-induced Marangoni flow with speed controlled by the incident angles and light intensities. More interestingly, the marble can alter the motion direction from toward (positive phototaxis) to away from (negative phototaxis) NIR light source by simply changing the light irradiation position from upper to lower semi-marbles due to the unique light absorbance/transmittance characteristics of the Fe3O4@OA NPs. This is dramatically different from previously reported light-driven active matters that only show single phototaxis. This finding provides an advanced strategy to remotely steer magnetic liquid marbles, which may stimulate the exploitation of their application in chemistry and biology fields at microscale, and enlightens a method for phototactic regulation of active matters.